General boating question?

I own one of those styrofoam sailboats which I recently fibergled and enjoy sailing a lot. Many times, when a motor boat nearby, especially a large boat starts up it's motor or revves to full power, I am able to hear a hissing sound that seems to eminate from the water and through the hull of my boat. Does anyone know what exactly might cause that hissing sound? (I know that it is not, I repeat, not anything with my boat as it has no motor.)